The emerging field of bioelectronics seeks to exploit biology in conjunction with electronics in a wider context encompassing, for example, micro or nanoscale biomaterials for information processing, information storage and actuators. A key aspect is the interface between biological materials and electronics since it defines the target, sensitivity, selectivity and speed of the device.
The detection of odorants has also been pursued through the development of electronic noses that are used for environmental monitoring, medical testing, and food and drink production. In the most sophisticated systems, a unique chemical fingerprint can be generated by an array of sensors and then identified by pattern-recognition techniques, such as the smell of a rose (Lundstrom, I., Nature, 406:682-3, 2000).
Attempts to measure odors with electronic instruments were made in the 1950s, but the modern field of artificial olfaction, according to Lundstrom (Lundstrom, I., Nature, 406:682-3, 2000), began in 1982 with the work of Persaud and Dodd (Persaud, K., and Dodd G., Nature. 299:352-5, 1982). They used a small array of gas-sensitive metal-oxide devices to classify odors. While there has been a steady increase in the number of systems using chemical sensor arrays, their success depends not only on the development of new sensor technologies, but also on the availability of powerful pattern-recognition software (Lundstrom, I., Nature, 406:682-3, 2000). This last aspect is particularly important for sensor arrays that produce a composite response for detecting targets that emit a characteristic array of odorants. However, these systems suffer from many limitations that are superseded by the olfactory cells in animals.
Olfactory receptor neurons (olfactory cells) are bipolar nerve cells that densely line the olfactory membrane in the recess of the nose, wherein odor receptor proteins that respond to odor molecules, called olfactory receptors, are expressed at high density. In olfactory cells, the chemical substances diffusing in the air from the stimulus source are detected by olfactory receptors and converted to neural signals. These neural signals are transmitted to the brain through the olfactory bulb (mitral cells or tufted cells) and the olfactory cortex such as the piriform cortex (pyramidal cells) and allow humans to sense odors. The interaction of odorants with olfactory receptors on the apical cilia of olfactory neurons is the first step in the perception of smell. The large number (e.g., approximately ˜350 in human and 1200 in dog) and structural diversity of the opsin-like GPCRs that function as olfactory receptors underlies the ability to detect and discriminate a vast number of volatile compounds (Buck, L. and Axel, R., Cell 65: 175-187, 1991; Fuchs, T. et al., Hum. Genet. 108: 1-13, 2001). Olfactory receptors interact with a diverse array of volatile molecules. It is widely accepted that every odorous molecule binds to several ORs and vise versa. This binding pattern generates a unique combinatorial code that generates a specific aroma for each odorant and enables the organism to distinguish it from other molecules. This system is highly sensitive and allows to discriminate between two protein isomers and at times even between two optical enantiomers.
Notwithstanding recent advances in bioelectronic sensing device, a quest for real time bio-sensing nanodevices with improved speed, precision and sensitivity still remains.